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10/27/2014
1
Chapter Outline
1
• 14.1 The Dynamics of Chemical Equilibrium
• 14.2 Writing Equilibrium Constant Expressions
• 14.3 Relationships between Kc and Kp Values
• 14.4 Manipulating Equilibrium Constant Expressions
• 14.5 Equilibrium Constants and Reaction Quotients
• 14.6 Heterogeneous Equilibria
• 14.7 Le Châtelier’s Principle
• 14.8 Calculations Based on K
• 14.9 Equilibrium and Thermodynamics
• 14.10 Changing K with Changing Temperature
Heterogeneous Equilibria
• Homogeneous equilibria - products and reactants
are all in the same phase
• Heterogeneous equilibria - products and reactants
are in different phases
CaO(s) + SO2(g) = CaSO3(s)
CaCO3(s) = CaO(s) + CO2(g)
K= CaO(s) [CO2(g)]
[CaCO3(s)]
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Pure Solids and Liquids do not Appear in
Equilibrium Constant Expressions
Kc = [SO2(g)]
CaCO3(s) = CaO(s) + CO2(g)
Kc = CaO(s) [CO2(g)]
[CaCO3(s)]
Do not appear in equil. const. expressions because
there is so much mass per unit volume that the amount
consumed is insignificant compared to aqueous or gas
phase reactants and products.
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Chapter Outline
5
• 14.1 The Dynamics of Chemical Equilibrium
• 14.2 Writing Equilibrium Constant Expressions
• 14.3 Relationships between Kc and Kp Values
• 14.4 Manipulating Equilibrium Constant Expressions
• 14.5 Equilibrium Constants and Reaction Quotients
• 14.6 Heterogeneous Equilibria
• 14.7 Le Châtelier’s Principle
• 14.8 Calculations Based on K
• 14.9 Equilibrium and Thermodynamics
• 14.10 Changing K with Changing Temperature
Le Chatelier’s Principle
If a system at equilibrium is perturbed (or
stressed), the position of the equilibrium shifts
in the direction that relieves the stress.
Henri Louis Le Chatelier
1. Effects of Adding or Removing
Reactants or Products
2. Effects of Changes in Pressure
and Volume
3. Effect of Temperature Changes
ratef = rater
?
?
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1. Effects of Adding or Removing
Reactants or Products
Industrial prep of H2(g): H2O(g) + CO(g) = H2(g) + CO2(g)
Removing CO2(g) CO2(g) + H2O(l) + K2CO3(aq) = 2 KHCO3(s)
Shifts 1st rxn towards
products ratef = rater
K = H2 [CO2]
H2O [CO]
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2. Effects of Changes in Pressure
and Volume • A rxn with gas-phase reactants or products may be perturbed by
changing the partial pressures of the gases
• A simple way to do this is to change the volume while keeping the
temperature constant
2 NO2(g) = N2O4(g) Kp =
PN2O4
(PNO2)2
NO2
NO2 NO2
NO2
NO2
NO2
N2O4 N2O4
N2O4
N2O4
N2O4
Halve the
volume NO2
NO2 NO2
NO2
NO2 NO2
N2O4 N2O4
N2O4 N2O4
N2O4 At const T and n,
according to Boyle’s
Law halving the
volume will double the
pressure.
2 NO2(g) = N2O4(g)
Kp =
PN2O4
(PNO2)2
= Y
X2
Qp = 2Y
(2X)2
• For clarity, substitute X and Y for the
partial pressures
• After halving the volume and doubling
the pressure, calculate Qp to determine
which direction the equilibrium will shift
= 2 Y
4 (X)2
= Y
2 X2
= 1
2 KP
• Q < Kp, so the amount of
products needs to increase in
order to re-attain equil
• So the rxn shifts towards
products
• Notice that there is a total of 2 moles of gas on the reactant side and 1 mole
total of gas on the product side
• Since there are fewer moles on the product side, shifting the rxn to the right
decreases the total number of moles in the reaction chamber
• When the total number of moles decreases, so does the pressure, and then
equilibrium is re-established
• Conversely, if the volume of the reaction chamber increases, then the rxn
would shift towards reactants in order to increase the total pressure
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3. Effect of Temperature Changes
• Changes in Temperature - Exothermic Reactions
2 SO2(g) + O2(g) 2 SO3(g) Ho = -180 kJ
Reaction gives off heat so THINK OF AS A PRODUCT
2 SO2(g) + O2(g) 2 SO3(g) + HEAT
Change Shifts the Equilibrium
Increase temperature (add product) left
Decrease temperature (remove product) right
The only perturbation that changes the value of
the equil const K. More in Section 14.10
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• Changes in Temperature - Endothermic Reactions
½ N2(g) + O2(g) NO2(g) Ho = +33.2 kJ
Reaction absorbs heat so THINK OF AS A REACTANT
HEAT + ½ N2(g) + O2(g) NO2(g)
Change Shifts the Equilibrium
Increase temperature (add reactant)
left Decrease temperature (remove reactant)
right
3. Effect of Temperature Changes
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Summary - Changes in Concentration
Summary - Changes in Pressure
and Volume
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Summary - Changes in Temperature
• Exothermic Reactions
Change Shifts the Equilibrium
Increase temperature (add product) left
Decrease temperature (remove product) right
• Endothermic Reactions
Reaction absorbs heat so THINK OF AS A REACTANT
Change Shifts the Equilibrium
Increase temperature (add reactant)
left Decrease temperature (remove reactant)
right
Reaction gives off heat so THINK OF AS A PRODUCT
Chapter Outline
18
• 14.1 The Dynamics of Chemical Equilibrium
• 14.2 Writing Equilibrium Constant Expressions
• 14.3 Relationships between Kc and Kp Values
• 14.4 Manipulating Equilibrium Constant Expressions
• 14.5 Equilibrium Constants and Reaction Quotients
• 14.6 Heterogeneous Equilibria
• 14.7 Le Châtelier’s Principle
• 14.8 Calculations Based on K
• 14.9 Equilibrium and Thermodynamics
• 14.10 Changing K with Changing Temperature
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Calculations Based on K
1.We want to determine whether a reaction mixture
has reached equilibrium (Sample Exercise 14.7)
2.We know the value of K and the starting
concentrations or partial pressures of reactant
and/or products, and we want to calculate their
equilibrium concentrations or pressures
K= [products]
m
[reactants]n
The “RICE” Table
N2(g) + O2(g) = 2 NO2(g) 1 1 2
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Algebra
x = −b ± b2−4ac
2a
ax2 + bx + c = 0
=
a b c
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